This is a follow up commit to 06eb21856f, which added QTWEBENGINEPROCESS_PATH
to the list of wrapped variables. Unfortunately it wouldn't be set, as its
value is a plain file rather than a directory, and the code only checked for
directories.
* guix/build/qt-build-system.scm (variables-for-wrapping): Define a file type
entry for each variable definition, and use it to determine if we should look
for directories versus plain files.
fed28a9632